Another NUUN vote. I put 1 NUUN tablet in a 24 oz bottle (it's supposed to be 1 tablet/16oz) and I carry another 24 oz bottle with water in it.
I bring some sort of food with me, depending on the ride and temperature. Fig newtons are good, cheese crackers...
I also often carry 1 tiny bag of Sports Beans. But I suspect that's mostly because I like them and I love the excuse to eat candy.

Sandy Earl, who just broke the women's 12 and 24 hr distance records, likes pizza. She says that someone can hand her a pizza as she pedals by and she can hold it and it gives her a punch.
Don't know what she likes on her pizza